1. Field of the Invention
This invention relates to a method for the production of a superconducting oxide tape having a Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 (Bi-2223) structure and a superconducting oxide tape produced by the method.
2. Description of the Prior Art
Research concerning superconducting oxides has advanced remarkably in both basic and applied fields in recent years. In the basic field, numerous papers have been published regarding the discovery of superconductors of novel compositions, the development of new methods of synthesis, the elucidation of superconduction mechanisms, etc. In the applied field as well, research on applications has expanded over a broad spectrum of materials ranging from electric and electronic materials to therapeutic materials. In numerous fields expectations are high regarding the development of oxide superconducting materials and the exhancement of their functions.
Such nonoxide type superconducting materials as niobium titanium alloys are currently utilized extensively as superconducting substances. Superconducting magnets, exceeding 15 teslas in capacity, are being manufactured with such superconducting substances. Owing to their low critical temperatures, however, these superconductors must be kept cooled with liquid helium, at huge cost of refrigeration.
Some of the superconducting oxide substances have critical temperatures exceeding the critical temperature of liquid nitrogen. If they could be practically applied, they would immensely reduce the cost of refrigeration. Thus, research on utilization of superconducting oxide substances for power storage, power transmission, generation of strong magnetic fields, etc. is being actively promoted. To realize such applications, it is necessary to reduce the superconducting oxide substances to wire rods or tapes.
The superconductor of the Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 (Bi-2212) structure, one of the Bi type superconductors, is manufactured by the partial melting technique into a tape which exhibits a very high critical current density in liquid helium. The present inventors earlier developed an improved method of producing a tape of the Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 structure based on the method mentioned above and a patent application has been filed thereon (U.S. patent application Ser. No. 08/873,090 which issued as U.S. Pat. No. 5,846,910). Their invention relating to the production of a superconducting polycrystalline wire rod from superconducting whiskers of the Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 structure has also been applied for patent (U.S. patent application Ser. No. 08/873,084). Since the critical temperature of the Bi-2212 superconductor is 80-90 K, only slightly different from the temperature of liquid nitrogen, the Bi-2212 superconducting oxide tape is hard to use at the temperature of the liquid nitrogen.
The wire rod of the Bi-2223 superconductor, which has a critical temperature as high as 110 K, has therefore been attracting attention. So far, the partial melting technique has not yet succeeded in producing a wire rod of the Bi-2223 superconductor. The Bi-2223 superconductor wire rod is almost always obtained by the solid-phase sintering technique. Since the Bi-2223 superconductor wire. rod is a polycrystalline substance, the factors thereof such as the intensity of superconducting bond at the grain boundary and the particle diameter of crystals directly affect the critical current density. Specifically, the critical current density rises as the strength of bond at the grain boundary and the size of crystal particles increase. Since the superconductor wire rod produced by the solid-phase sintering technique is limited as regards the superconducting bond intensity and the crystal particle diameter, the improvement thereof in critical current density is likewise limited.
It is the main object of this invention to provide a practical method for the production of a Bi-2223 superconducting oxide wire rod or tape by the partial melting technique.